Traditional methods of developing a signal relating to the output current of a power supply are often either inaccurate, expensive, or dissipate excessive power.
One such method uses a small resistor in series with the power supply output lead (e.g. commonly, in series with the output filter inductor). The voltage across this resistor is directly proportional to the output current. This method has some disadvantages. For high current outputs this technique dissipates significant power. If the signal developed across the sense resistor is small in order to reduce power consumption, an expensive precision amplifier must be used to accurately amplify the signal to workable levels.
The disadvantages of expensive electronics and power dissipation may be over come to some degree by inferring the output current of an isolated switch mode power supply from a power switch current signal. Making use of the power switch current signal is advantageous since this signal is often already present for the purpose of power switch protection and power unit dynamic control.
Direct resistive measurement of the current through the power switch has the advantage of reduced power dissipation in the sense resistor element if the power switch current sensed is that of the primary switch of a step-down converter, or if the switch current is sensed using a current sense transformer technique. However, sensing the switch current produces a time varying signal whose average is often equal to the power unit's input current, not its output current. The output current must be inferred from the switch current signal. A common technique involves placing a standard peak detector circuit across the sense signal. This technique has two major sources of error: one is that this method senses the peak of the sense signal, which is corrupted by AC transformer magnetizing current and output inductor ripple current. Another source of error is the forward drop of the peak detector diode, which is often a significant fraction of the sensed signal's amplitude.
A technique used to improve the accuracy of the output current estimation from the power switch current signal uses a timing circuit that attempts to determine the midpoint of the interval in which current is flowing through the power switch, and then sampling the sense signal at that instant. This technique is complicated, and is prone to timing errors when used in conjunction with high frequency, short period power converters that operate with substantially varying duty ratios of switch conduction.